Jenson Button thinks McLaren will struggle to challenge for pole position at the Korean Grand Prix but is more confident about his car's race pace.

Button finished the second practice session fourth fastest and 0.387s off the pace of Sebastian Vettel's best lap, but was roughly 0.5s faster on heavy-fuel runs. After Vettel and Mark Webber locked out the front row in Suzuka, Button said it would again be difficult to match the Red Bull's pace over one lap.

"They've got that 0.4s again in practice and I think they're going to be very difficult to beat in qualifying; they seem to have that strength back," he said. "Our race pace today seemed to be competitive but over one lap we're still lacking so it's an area we need to work on. But I feel the direction that we've gone with the car is positive so there is still more to come."

Button's super-soft run was slightly compromised by a slower car.

"I had a little bit of traffic in the last sector behind Maldonado. I don't think it was a lot but my last sector was slower than I managed on the prime tyre. For us near the front, I think we're getting the prime to work even though it's a high temperature tyre."

But Button said he still needed to find a better balance between straight-line pace and downforce in the corners as he was hitting the rev limiter quite early on the straights.

"If you have a look we're very slow in a straight line," he added. "In sector one we are about 8km/h down on everyone else. So it's a tricky one to know which direction to go with that and how much downforce to run on the rest of the circuit. But we have a lot of data, a good long run and some good low-fuel runs to work out where we stand. Our tyre man will look at the data and figure out where we stand for tomorrow."